The removal of sulfur from petroleum fractions represents a major challenge in petroleum refining. Sulfur compounds, such as hydrogen sulfide, mercaptans, thiophenes, and elemental sulfur are impurities in petroleum fractions. If these impurities not removed from petroleum fractions, these sulfur impurities will corrode process equipment, impart poor color and odor properties to products, and poison downstream catalytic processes. The environmental impact of sulfur in various refining products may also be significant. For example, even though the current level of sulfur in motor gasoline is limited to less than 0.10 wt %, there are indications that even this level is not low enough to meet future standards for emissions from automobile exhaust. In a modem U.S. refinery, roughly over 50% of the gasoline pool comprises cracked gasoline produced from a fluid catalytic cracking (FCC) process. This makes FCC gasoline a major part of the gasoline product pool in the United States. Because FCC gasoline is produced from the heaviest and often the most sulfur-contaminated streams in the refinery, it provides a large portion of the sulfur in the gasoline product pool. The reduction of sulfur in gasoline, particularly to levels such as 300 ppm-wt as required to comply with environmental regulations are said to reduce automobile exhaust emissions of carbon monoxide, nitrogen oxides and hydrocarbons as well as sulfur oxides.
Naphthas and other light fractions such as heavy cracked gasoline may be hydrotreated by passing the feed over a hydrotreating catalyst at elevated temperature and somewhat elevated pressure in a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII and a Group VI element, such as cobalt and molybdenum, on a substrate such as alumina. After the hydrotreating operation is complete, the product may be fractionated, or simply flashed, to release the hydrogen sulfide and collect the now sweetened gasoline.
Cracked naphtha, as it is produced from the FCC and without any further treatments such as purifying operations, has a relatively high octane number as a result of the presence of olefinic components. In some cases, this fraction may make a significant contribution to product octane. Hydrotreating of any of the sulfur-containing fractions which boil in the gasoline boiling range causes a reduction in the olefin content and, consequently, a reduction in the octane number. As the degree of desulfurization increases, the octane number of the normally liquid gasoline boiling range product decreases. Some of the hydrogen may also cause some hydrocracking as well as olefin saturation, depending on the conditions of the hydrotreating operation further lowering the octane of the fraction.
U.S. Pat. No. 2,514,997 to Floyd discloses a process for the removal of sulfur from a non-aromatic hydrocarbon feed using a solvent comprising a poly-olefin glycol having a molecular weight in the range of about 400 to 4,000 to produce a raffinate phase being substantially sulfur free.
U.S. Pat. No. 3,957,625 to Orkin discloses that the sulfur impurities tend to concentrate in the heavy portion of the cracked gasoline fraction. Orkin discloses a process wherein the cracked gasoline is fractionated to separate the heavy fraction of the catalytically cracked gasoline and hydrotreating the heavy fraction. Orkin does not attempt to recover octane lost in the hydrotreating of the heavy fraction.
Processes for removing sulfur without reducing the octane of the FCC gasoline and similar streams are disclosed in U.S. Pat. Nos. 5,298,150 and 5,290,427 Fletcher et al. wherein sulfur containing fraction of the FCC gasoline is desulfurized and the desulfurized fraction is contacted with an acidic catalyst to restore the octane of the desulfurized fraction. This process requires an energy intensive fractionation of the entire cracked gasoline stream to obtain the higher boiling fraction which contains the bulk of the sulfur impurities in the cracked gasoline.
U.S. Pat. No. 2,634,230 to Arnold et al. discloses a process for the desulfurization of high sulfur olefinic naphtha which Arnold teaches is the most difficult to desulfurize or otherwise refine by conventional methods. In the process 2,4-dimethyl sulfolane is employed to extract sulfur from a highly olefinic naphtha, such that the solvent does not affect separation between olefins and paraffins, to provide a sulfur lean raffinate phase and a sulfur rich extract. Both the raffinate and extract phases are distilled to remove the solvent and provide a dewatered raffinate and a dewatered extract. The dewatered extract is catalytically desulfurized and the resulting desulfurized extract is blended with the dewatered raffinate to provide a desulfurized naphtha product. Although Arnold avoids a costly fractionation step on the cracked gasoline stream, Arnold's process includes the costly distillation of both the extract and the raffinate streams to recover the sulfolane solvent.
U.S. Pat. No. 2,664,385 to Wolff et al. discloses a process for the extraction of organic sulfur compounds from a mixture thereof with hydrocarbons wherein the mixture is contacted with an ester of a thiosulfonic acid containing 2 to 20 hydrocarbons per molecule to provide a raffinate phase and an extract phase comprising the ester and the organic sulfur compound.
U.S. Pat. No. 2,956,946 to King et al. relates to a solvent extraction process for the removal of acid oils such as alkylated phenols, aerosols, xylenols, thiophenols and the like from petroleum distillates boiling between about 100.degree. F. and about 900.degree. F. by employing an ethylene glycol monoalkylamine ether to extract the acid oils and recover an acid-free raffinate. King et al. discloses that the acid oils were extracted by the solvents in preference to aromatics regardless of the conditions employed. King et al. discloses a process whereby the feedstream containing the acid oils is contacted in an extraction zone with a solvent to provide a raffinate stream and a rich solvent stream. The raffinate stream is water washed to provide a treated petroleum distillate and a water and solvent stream. The water and solvent stream is passed to a settling zone where the water and solvent stream is contacted with the rich solvent stream to provide an aromatics fraction and a second rich solvent stream. The second rich solvent stream is passed to a distillation column to separate water from the second rich solvent stream to provide an anhydrous rich solvent stream. The anhydrous rich solvent stream is passed to a vacuum tower to separate the acid oils and to provide a lean solvent stream. The lean solvent stream and a portion of the anhydrous rich solvent stream are returned to the extraction zone.
U.S. Pat. No. 2,792,332 to Hutchings discloses a process for the removal of aromatics and sulfur compounds from a feedstream comprising heavy naphtha, aromatics and sulfur compounds wherein the feedstream comprising heavy naphtha is contacted in a first extraction column with a solvent combination comprising isopropyl alcohol and polyethylene glycol having a molecular weight of about 600, in volume percent ratio of about 70 to 30 of glycol to alcohol, respectively, to obtain a concentrated aromatic fraction and a paraffinic-naphthenic raffinate. Hutchings recycles the raffinate for reprocessing with the feedstream. The raffinate is first distilled to remove the alcohol, and the resulting alcohol depleted raffinate is water washed to remove traces of the polyethylene glycol 600 and then dried. The extract phase is similarly processed to first remove the alcohol by distillation and the alcohol-free extract is steam distilled to recover an aromatic product and to provide an aromatic-free polyethylene glycol/water stream. The polyethylene glycol/water stream is then passed to a solvent recovery tower to distill off the remaining water. In a second extraction column, the concentrated aromatic fraction is contacted with pure polyethylene glycol to recover an aromatic extract and the aromatic extract is steam distilled to provide a purified aromatic product comprising aromatic sulfur-type compounds.
U.S. Pat. No. 4,781,820 to Forte and U.S. Pat. No. 4,498,980 to Forte et al. disclose processes for the separation of aromatic and non-aromatic hydrocarbons from a mixed hydrocarbon feed wherein the feedstream is contacted with a solvent comprising a polyalkylene glycol and a co-solvent comprising a glycol ether. The U.S. Pat. Nos. 4,781,820 and 4,498,980 are hereby incorporated by reference.
In any case, regardless of the mechanism by which it happens, the decrease in octane which takes place as a consequence of sulfur removal by hydrotreating creates a conflict between the growing need to produce gasoline fuels with higher octane number and--because of current ecological considerations - the need to produce cleaner burning, less polluting fuels, especially low sulfur fuels.
Processes are sought for the efficient removal of sulfur compounds from FCC gasoline and similar petroleum refinery streams without the loss of gasoline octane yield and quality, and at a minimum reprocessing cost.